I’ve used Daphnia these last few years to run undergraduate research projects, and school outreach stuff, and most recently undergrad pharmacology practical classes. We usually measure their heart rate, since you can see the heart through the carapace. (It’s actually near what you would probably call the animal’s ‘back’, if you look carefully at RPG’s film, but you have to look hard as the heart wall is so thin that the heart is nearly transparent). You can measure other stuff too, like beat rate of the ‘antennae’ and even pulsations of the gut.